More foreign oil facilities are shutting down in Nigeria as a result of continuing ethnic violence in the country's Niger River Delta. The move is further cutting production from Nigeria, the world's sixth-largest producer of oil.

The giant British-Dutch oil company Shell said it is evacuating at least four of its production facilities in the Niger River Delta, citing concern for the safety of its employees.

Officials said the shutdowns would cut Shell's oil production in Nigeria by 320,000 barrels a day.

Shell's action follows similar moves by the French-based TotalFinaElf and the U.S. oil giant Chevron Texaco, which announced Sunday it is closing down nearly all of its onshore and offshore operations in the country.

Officials say Nigeria's oil production has been slashed by one-third since hostilities began earlier this month between ethnic Ijaws and Nigerian government forces.

Ijaw militants attacked oil facilities to protest what they charge are government moves to re-draw electoral districts before next month's presidential elections.

The Nigerian government sent hundreds of troops to the region to quell the violence. The military intervention has prompted accusations by Ijaws that the troops have opened fire on their villages.

The Nigerian army denies the accusation, saying troops have sought to restore order with a minimum of force.

Ijaw leaders are threatening to attack more facilities if their demands for greater political representation are not met.

The violence has spurred further increases in world oil prices, which are already high due to the war in Iraq.